Conotoxins Targeting Nicotinic Acetylcholine Receptors: An Overview

Marine snails of the genus Conus are a large family of predatory gastropods with an unparalleled molecular diversity of pharmacologically active compounds in their venom. Cone snail venom comprises of a rich and diverse cocktail of peptide toxins which act on a wide variety of ion channels such as voltage-gated sodium- (NaV), potassium- (KV), and calcium- (CaV) channels as well as nicotinic acetylcholine receptors (nAChRs) which are classified as ligand-gated ion channels. The mode of action of several conotoxins has been the subject of investigation, while for many others this remains unknown. This review aims to give an overview of the knowledge we have today on the molecular pharmacology of conotoxins specifically interacting with nAChRs along with the structure–function relationship data

Adiponectin increases insulin content and cell proliferation in MIN6 cells via PPAR gamma-dependent and PPAR gamma-independent mechanisms

Aims Adiponectin is an important adipokine whose levels are decreased in obesity despite increases in adipocyte mass. Studies in animal models implicate adiponectin as an insulin sensitizer in skeletal muscle and liver. Thiazolidinediones (TZDs) are insulin sensitizers and ligands for peroxisome proliferator-activated ? receptors (PPAR gamma) and these receptors are expressed in beta cells where their activation promotes cell survival. We hypothesize that adiponectin promotes beta cell survival by activating PPAR gamma. Methods We used MIN6 cells to investigate the effect of adiponectin on PPAR gamma expression, beta-cell proliferation, insulin synthesis and insulin secretion. Results We demonstrate that MIN6 cells contain adiponectin receptors and that adiponectin activates PPAR gamma mRNA and protein expression. This increase in PPAR gamma expression is blocked by the PPAR gamma antagonist, GW9662, indicating a transcriptional feedback loop involving PPAR gamma activation of itself. Adiponectin causes a significant increase in insulin content and secretion and this occurs also via PPAR gamma activation due to the inhibitory effect of GW9662. Adiponectin also promotes MIN6 cell proliferation, however, this effect is independent of PPAR gamma activation. Conclusions Our results identify novel roles for the adipokine, adiponectin, in beta-cells function. Adiponectin upregulates PPAR gamma expression, insulin content and insulin secretion through PPAR gamma-dependent mechanisms. Reductions in circulating adiponectin levels in obese individuals could therefore result in negative effects on beta-cell function and this may have direct relevance to beta-cell dysfunction in type 2 diabetes

The Glutamate Agonist Homocysteine Sulfinic Acid Stimulates Glucose Uptake through the Calcium-dependent AMPK-p38 MAPK-Protein Kinase C ζ Pathway in Skeletal Muscle Cells*

Homocysteine sulfinic acid (HCSA) is a homologue of the amino acid cysteine and a selective metabotropic glutamate receptor (mGluR) agonist. However, the metabolic role of HCSA is poorly understood. In this study, we showed that HCSA and glutamate stimulated glucose uptake in C2C12 mouse myoblast cells and increased AMP-activated protein kinase (AMPK) phosphorylation. RT-PCR and Western blot analysis revealed that C2C12 expresses mGluR5. HCSA transiently increased the intracellular calcium concentration. Although α-methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor antagonist, blocked the action of HCSA in intracellular calcium response and AMPK phosphorylation, 6-cyano-7-nitroquinoxaline-2,3-dione, an AMPA antagonist, did not exhibit such effects. Knockdown of mGluR5 with siRNA blocked HCSA-induced AMPK phosphorylation. Pretreatment of cells with STO-609, a calmodulin-dependent protein kinase kinase (CaMKK) inhibitor, blocked HCSA-induced AMPK phosphorylation, and knockdown of CaMKK blocked HCSA-induced AMPK phosphorylation. In addition, HCSA activated p38 mitogen-activated protein kinase (MAPK). Expression of dominant-negative AMPK suppressed HCSA-mediated phosphorylation of p38 MAPK, and inhibition of AMPK and p38 MAPK blocked HCSA-induced glucose uptake. Phosphorylation of protein kinase C ζ (PKCζ) was also increased by HCSA. Pharmacologic inhibition or knockdown of p38 MAPK blocked HCSA-induced PKCζ phosphorylation, and knockdown of PKCζ suppressed the HCSA-induced increase of cell surface GLUT4. The stimulatory effect of HCSA on cell surface GLUT4 was impaired in FITC-conjugated PKCζ siRNA-transfected cells. Together, the above results suggest that HCSA may have a beneficial role in glucose metabolism in skeletal muscle cells via stimulation of AMPK